(1) Field of the Invention
This invention relates to a process for the preparation of a silicon nitride powder excellent in the sintering activity.
(2) Description of the Prior Art
As is well-known, silicon nitride (Si.sub.3 N.sub.4) is advantageous over conventional ceramic products in the following points: (1) the mechanical strengths and hardness are very high, and the strengths at high temperatures is high, (2) the thermal shock resistance is high and the fire resistance is high, (3) the chemical stability is good and the corrosion resistance is high, and (4) the electrically insulating property is good. Accordingly, sintered silicon nitride are broadly used in various fields, for example, as high-grade refractory materials for metal smelting, ceramic industry and machine industry, other refractory materials, wear resistant materials and electrically insulating materials.
Recently, silicon nitride has attracted attentions in the art as staring materials for high temperature articles for which high strengths, high heat resistance and high mechanical shock resistance are required, such as gas turbines.
Since silicon nitride is inherently difficult to sinter, various attempts have heretofore been made to improve the sintering property of silicon nitride, but no satisfactory method for improving the sintering property of silicon nitride has been developed. According to the conventional techniques, if the sintering activity is improved, the high temperature strengths are reduced, and it is difficult to obtain a sintered product which is satisfactory in both the sintering activity and the high temperature strengths. For example, if silicon nitride powders prepared according to various conventional processes are sintered under ordinary conditions by pressureless sintering, shrinkage due to sintering is hardly caused, and therefore, the density and strengths of the obtained sintered products are very low and they cannot be used for the intended objects at all. Highly dense materials may be obtained from materials which are difficult to sinter, such as silicon nitride and SiC, by hot pressing under a super high pressure without using a sintering aid as in case of the synthesis of diamond. However, since this process is carried out under a super high pressure, the sintering cost is expensive and, from the economical and industrial viewpoints, the process cannot be adopted. Accordingly, 10 to 20% by weight of an oxide such as MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 or ZrO.sub.2 has been added as a sintering aid to a silicon nitride powder. A sintered material of silicon nitride obtained by adding such a sintering aid is characterized in that a low-melting-point phase is formed in the crystal grain boundary of silicon nitride and hence, sintering is promoted and the density is increased. However, formation of this low-melting-point phase results in reduction of the high temperature strengths. Therefore, it is difficult to obtain a sintered product which is satisfactory in both the high density and the high temperature strength.